The introduction of intelligent workstations, which are usually LAN-attached, has changed the data processing paradigm from centralized host computing to distributed processing. Also, with the growth of distributed processing, the need for LAN interconnection and the growing use of graphics and images has lead to exponentially increasing network traffic. Furthermore, not only the demand for connectivity has changed, but also the technology to provide networking facilities has been subjected to important changes. The introduction of digital and fiber technologies provides faster and more reliable communication but requires networking techniques which are able to efficiently operate at higher speeds. In order to meet this requirement, the concept of fast packet switching has been developed.
Fast packet switching, often used to refer to Frame Relay, is a generic term that relates to packet switching technologies that omit most of OSI model layer 2 processing and all of layer 3 to 7 processing to achieve higher data throughput. Because fast packet switching such as Frame Relay operates below layer 3 of the OSI model, it is easy to run multiple protocols over it, in particular the IP protocol.
The Frame Relay network provides a number of Permanent Virtual Circuits (PVC) that form the basis for the connections between stations attached to the network and that allow data exchange between these stations. The resulting set of interconnected devices is the Frame Relay group which may be either fully interconnected to form a fully meshed network, or only partially interconnected to form a partially meshed network. In either case, each PVC is uniquely identified at each Frame Relay interface by a Data Link Connection Identifier (DLCI). Such a DLCI, which is therefore different on either end of the PVC, has strictly local significance at each interface.
A fully meshed Frame Relay network is not subject to connectivity problems. In the IP configuration, the whole network is seen as a single IP subnet. This configuration has no limitation since any router can reach all other routers, except that it requires a high number of PVCs, which number increases as each a new router is added to the network.
Generally, the mapping between the IP addresses of the routers in the IP subnet and the DLCIs to be used by a router to reach each one of the other routers is achieved by using an inverse ARP (Address Routing Packet) table associated with the router. The dynamic method for updating the inverse ARP table consists for a router in sending or receiving requests over a PVC, bearing in mind that the known hardware address is the DLCI corresponding to the router end. When receiving either a reply to an ARP request or a request over the PVC, the router can associate, in its inverse ARP table, the IP address (as entry) of the device at the other end of the PVC with the DLCI being used. Since a fully meshed network is seen as a single IP subnet and since any router has PVC connectivity to all other routers in such a network, it can dynamically map the remote IP address-to-DLCI using the inverse ARP method.
Partially meshed networks can be made of several IP subnets wherein one router, the hub, has a PVC for all other routers of the subnet, the spokes. In such a case, spoke to spoke connectivity is resolved via IP subnet to subnet connectivity, which is the normal IP routing process. When a spoke wants to reach another spoke of another subnet, it will use its routing table which indicates a route via the hub. The problem with this method is that it requires a different IP subnet per PVC. This can be a problem in case of IP address exhaustion. It also creates very large routing tables, because of the number of new subnets, which causes memory problems inside the routers along with high bandwidth utilization between the links when exchanging the routes for these subnets. Partially meshed networks can also be made of one single subnet. In that case, dynamic inverse table does not permit resolution of the spoke to spoke connectivity problem.
The solution to the above problems consists in doing for each spoke a manual static mapping instead of using the dynamic inverse ARP. This means that the inverse ARP table is manually configured with the IP addresses of all the spokes and the corresponding DLCIs. Unfortunately, such a solution which has to be achieved on all the spokes, can become very heavy and difficult when many spokes are present in the network.